In most printing presses, sheet-fed rotary printing mechanisms are used. Each sheet-fed rotary printing mechanism integrates one or more rotating components, such as cylindrical sheet guides in order to process the sheets and pass them among the different assemblies of a printing press. For example, most high-speed, sheet-fed rotary printing presses are provided with an impression drum. Cylinders which permit the press to print on two sides of a sheet, and which may be used for single-sided, multi-color printing are commonly used.
In order to retain sheets on the rotating components and in order to allow the transferring of sheets to and from the rotating components, grippers are usually used. The grippers are needed in order to facilitate the loading and unloading of sheets to and from the rotating components. Transmission elements and adjusting mechanisms are usually needed in addition to the grippers in order to coordinate among the printing press assemblies during the loading and unloading of the sheets and in order to allow different movement patterns of the sheets within the printing press. A turning impression drum, for example, is usually designed with spaces to receive grippers and transmission elements and has a cylindrical sheet support surface that is operative to guide the sheets, which extends over more than half the surface of the turning impression drum.
A common rotating component, such as an impression drum, is rotated by a drive unit having a motive mechanical force, such as an electric motor. Usually, the motive mechanical force actuates a drive shaft that supports and rotates the rotating component. As described above, the rotating component usually uses grippers for facilitating the loading and unloading of sheets. Usually, the drive unit or an additional drive unit supports an opening and closing of the grippers. The opening and closing change the position of the grippers in relation to the main body of the rotating component, with the result that the mass distribution of the rotating component is changed. One inherent property of such changes is load imbalance. Even a very small imbalance in the otherwise symmetrical mass distribution about the drive shaft may cause bending vibrations thereto and to the rotating component it supports. The rotating component is subjected to changes in the mass distribution during the actuation of the grippers. These changes increase the load imbalance and cause the rotating component to be subjected to disruptive mechanical forces, such as retroactive torque effect.